Well, I was wondering this too and I noticed that on the powebooks the hinges nearly take up all of the backside of the notebook and on the ibooks it takes about a third right in the dead center for the same screen size as the pbooks. This makes it seem weaker but I have had no problems with it. It is tight (wont slide close or open by itself) and not hard to open at all. (barely any force applied once the release button is pushed will open it). So maybe it is even a better thing that it doesn't require as much wires and such going into the screen as the powerbook does (justifying their big hinges). Im not too sure.